JavaScript Best Practices

Avoid global variables, avoid new, avoid ==, avoid eval()

Avoid Global Variables


Minimize the use of global variables.

This includes all data types, objects, and functions.

Global variables and functions can be overwritten by other scripts.

Use local variables instead, and learn how to use closures.

Always Declare Local Variables


All variables used in a function should be declared as local variables.

Local variables must be declared with the var keyword, otherwise they will become global variables.

Strict mode does not allow undeclared variables.

Declarations on Top


It is a good coding practice to put all declarations at the top of each script or function.

This will:

// Declare at the beginning
var firstName, lastName, price, discount, fullPrice;

// Use later
firstName = "John";
lastName = "Doe";

price = 19.90;
discount = 0.10;

fullPrice = price * 100 / discount;

This also goes for loop variables:

// Declare at the beginning
var i;

// Use later
for (i = 0; i < 5; i++) {

By default, JavaScript moves all declarations to the top (JavaScript hoisting).

Initialize Variables


It is a good coding practice to initialize variables when you declare them.

This will:

// Declare and initiate at the beginning
var firstName = "",
    lastName = "",
    price = 0,
    discount = 0,
    fullPrice = 0,
    myArray = [],
    myObject = {};

Initializing variables provides an idea of the intended use (and intended data type).

Never Declare Number, String, or Boolean Objects


Always treat numbers, strings, or booleans as primitive values. Not as objects.

Declaring these types as objects, slows down execution speed, and produces nasty side effects:


var x = "John";             
var y = new String("John");
(x === y) // is false because x is a string and y is an object.
Try it Yourself

Or even worse:


var x = new String("John");             
var y = new String("John");
(x == y) // is false because you cannot compare objects.
Try it Yourself

Don't Use new Object()



var x1 = {};           // new object
var x2 = "";           // new primitive string
var x3 = 0;            // new primitive number
var x4 = false;        // new primitive boolean
var x5 = [];           // new array object
var x6 = /()/;         // new regexp object
var x7 = function(){}; // new function object
Try it Yourself

Beware of Automatic Type Conversions


Beware that numbers can accidentally be converted to strings or NaN (Not a Number).

JavaScript is loosely typed. A variable can contain different data types, and a variable can change its data type:


var x = "Hello";     // typeof x is a string
x = 5;               // changes typeof x to a number
Try it Yourself

When doing mathematical operations, JavaScript can convert numbers to strings:


var x = 5 + 7;       // x.valueOf() is 12,  typeof x is a number
var x = 5 + "7";     // x.valueOf() is 57,  typeof x is a string
var x = "5" + 7;     // x.valueOf() is 57,  typeof x is a string
var x = 5 - 7;       // x.valueOf() is -2,  typeof x is a number
var x = 5 - "7";     // x.valueOf() is -2,  typeof x is a number
var x = "5" - 7;     // x.valueOf() is -2,  typeof x is a number
var x = 5 - "x";     // x.valueOf() is NaN, typeof x is a number
Try it Yourself

Subtracting a string from a string, does not generate an error but returns NaN (Not a Number):


"Hello" - "Dolly"    // returns NaN
Try it Yourself

Use === Comparison


The == comparison operator always converts (to matching types) before comparison.

The === operator forces comparison of values and type:


0 == "";        // true
1 == "1";       // true
1 == true;      // true

0 === "";       // false
1 === "1";      // false
1 === true;     // false
Try it Yourself

Use Parameter Defaults


If a function is called with a missing argument, the value of the missing argument is set to undefined.

Undefined values can break your code. It is a good habit to assign default values to arguments.


function myFunction(x, y) {
    if (y === undefined) {
        y = 0;
Try it Yourself

End Your Switches with Defaults


Always end your switch statements with a default. Even if you think there is no need for it.


switch (new Date().getDay()) {
    case 0:
        day = "Sunday";
    case 1:
        day = "Monday";
    case 2:
        day = "Tuesday";
    case 3:
        day = "Wednesday";
    case 4:
        day = "Thursday";
    case 5:
        day = "Friday";
    case 6:
        day = "Saturday";
        day = "Unknown";
Try it Yourself

Avoid Using eval()


The eval() function is used to run text as code. In almost all cases, it should not be necessary to use it.

Because it allows arbitrary code to be run, it also represents a security problem.